1. Field of the Invention
This invention relates to a method for the regeneration of a denitration catalyst.
2. Description of the Related Art
In order to remove nitrogen oxides (hereinafter referred to as NO.sub.x) produced in boilers and various combustion furnaces for the purpose of preventing air pollution, a catalytic ammonia reduction process wherein ammonia is used as a reducing agent and nitrogen oxides are decomposed to nitrogen and water by contact with a catalyst is being widely employed. Most of the NOx removal catalysts currently used for practical purposes are honeycomb-shaped catalysts which have through-holes of square cross section in 1-10 order to prevent clogging with dust present in exhaust gas and increase the gas contact area. With respect to catalyst components, titanium oxide is highly suitable for use as a principal component, and vanadium, tungsten and the like are commonly used as active components. Thus, TiO.sub.2 --WO.sub.3 binary catalysts and TiO.sub.2 --V.sub.2 O.sub.5 --WO.sub.3 ternary catalysts are popularly used. The catalytic power of such a denitration catalyst tends to be gradually reduced with service time, and the cause for the reduction in catalytic power varies according to the type of the used in the source of exhaust gas (e.g., boiler).
For example, in the case of exhaust gas from an oil-fired boiler, sodium contained in the dust present in exhaust gas is chiefly deposited on the catalyst and causes a reduction in catalytic power. In the c ase of exhaust gas from a coal-fired boiler, calcium contained in the dust present in exhaust gas is chiefly deposited on the catalyst surfaces and reacts with sulfuric anhydride present in the exhaust gas to form calcium sulfate. This calcium sulfate covers the catalyst surfaces and hinders NO and NH.sub.3 gases from diffusing into the interior of the catalyst, resulting in reduced catalytic power. It has conventionally been known that catalysts having reduced catalytic power attributable to these causes can be effectively regenerated by cleaning them with water or an aqueous solution of hydrochloric acid.
However, the conventional cleaning method using water or an aqueous solution of hydrochloric acid exhibits little regenerative effect on some catalysts. No effective method for the regeneration of such catalysts has been known in the prior art.